This invention relates to the manufacture of polymer contact lenses in an environment that requires the presence of oxygen at the optical surfaces of the contact lens mold in which the contact lens material is reacted.
The molding of hydrophilic contact lenses is known. Various processes are disclosed in U.S. Pat. No. 4,495,313, to Larsen; U.S. Pat. No. 4,640,489 to Larsen, et al.; U.S. Pat. No. 4,680,336 to Larsen et al.; U.S. Pat. No. 4,889,664 to Larsen et al.; and U.S. Pat. No. 5,039,459 to Larsen et al., all of which are assigned to the assignee of the present invention.
These prior art and other references generally disclose a contact lens production process wherein each lens is molded from a reaction mixture, which can be a reactive monomer or prepolymer mixture. The molding is done by a casting process in which the mixture to be polymerized is deposited into one first mold half or section, often referred to as a front curve, then a second mold half or section, often referred to as a back curve is assembled onto the first mold half, and the assembled mold sections are subjected to conditions resulting in polymerization or reaction of the mixture into a contact lens having the shape of the cavity formed between the two mold halves. These mold halves are usually formed from thermoplastic materials transparent to uv radiation such as polystyrene or polypropylene.
If prior to assembly, the mold halves are exposed to oxygen, the polymerization process may be inhibited to such an extent that the contact lenses will not have the desired physical properties. It is known that this is due to the O2 being adsorbed onto and absorbed into the plastic mold halves. It is known that O2 on and in the plastic halves adversely affects the polymerization of the lens material. The effect of O2 on the photopolymerization process is that it strongly inhibits radical-induced polymerization. Polymerization is suppressed until O2 has been consumed by reaction with radicals until the monomer is able to compete successfully with O2 for initiator radicals. Two types of systems have been identified: closed and open. Both types of systems apply to the present invention.
In the closed system, no O2 or a fixed amount of O2 is initially present in the system and polymerization proceeds appreciably after an induction period, during which the O2 is consumed by radicals. In the open system, O2 diffuses into the system and polymerization occurs only if sufficient radicals are generated to successfully compete with the O2. Open systems typically are systems that are open to air.
Exposing mold halves to O2 before assembly of the mold halves leads to a xe2x80x9cclosed-openxe2x80x9d system during polymerization. When the system is open, O2 absorbs onto the surface and absorbs into the mold, thus creating an O2 reservoir. When the mold is assembled (closed), after the induction period when O2 in the monomer and on and in the mold halves is consumed, polymerization proceeds in the lens bulk. The effect on lens properties is dependent on the amount of O2 absorbed into the mold prior to assembly.
The effect of O2 absorbed onto and into the mold on photopolymerization of the reaction mixture is expected to disrupt polymerization at the lens surface, i.e. to cause differential polymerization at the lens surface relative to the lens bulk. This disruption causes more loose polymer ends at the surface due to (premature) termination of polymerization by O2. These shorter chain polymers at the surface of the lens tend to have lower cross link density, less chain entanglement, and more tackiness than the polymer chains in the bulk of the lens. These factors result in reduced mechanical strength and increased water content at the lens surface relative to these properties in the lens bulk.
Under oxygen-free molding conditions, lenses are isotropic in nature. As O2 is introduced to the lens surface and not to the lens bulk during polymerization, lenses become less isotropic in nature and more anisotropic, and control of final lens properties within specified tolerance ranges is compromised.
To reduce the deleterious effect of O2, contact lens manufacture has been carried out in a reduced O2 environment, and/or the reaction mixture is treated to remove dissolved O2 prior to polymerization. In manufacturing, this has resulted in the use of techniques such as physical enclosure of the process and use of large quantities of nitrogen to blanket the assembly and pre-assembly areas. This technique includes the plastic mold halves within the blanketed area since the boundary layer of gases on the plastic surfaces will include O2 if not so protected. Typically, the percent O2 in the atmosphere surrounding the plastic molds halves is monitored and kept below 0.5 percent, the other 99.5 percent of the atmosphere is the inert gas. For example, see U.S. Pat. No. 5,555,504.
The prior art discloses that the amount of oxygen exposure must be limited or avoided to prevent the deleterious effects that the exposure to oxygen has on the manufacture of contact lenses. Various techniques for reducing the deleterious effects of O2 on the polymerization of contact lenses are found in the following U.S. patents:
EP Appln. No. 95937446.3 discloses a process in which plastic molds are treated prior to dosing with the reactive monomer mix to remove substantially all of the O2. The removal of the O2 can be accomplished by contacting the mold pieces with an inert gas or by using a vacuum. Molds that were not treated to remove the O2 provided contact lenses with high percentages of defects.
This invention addresses the need to increase the yields in contact lens manufacturing lines by reducing the percentage of contact lenses which are rejected due to defects.
This invention provides a method of manufacturing a contact lens comprising the step of:
dosing a contact lens mold comprising optical mold surfaces with an oxygen-sensitive reaction mixture, wherein said optical surfaces of said contact lens mold have between from 0.13xc3x9710xe2x88x929 to 2.6xc3x9710xe2x88x929 moles/cm2 O2 available to interfere with the reaction of said reaction mixture.
This invention further provides a method of manufacturing a contact lens comprising the steps of:
dosing a contact lens mold comprising optical mold surfaces with a reaction mixture, and sealing said reaction mixture and said optical mold surfaces away from a gaseous environment, wherein said optical surfaces of said contact lens mold were exposed to a gaseous environment comprising greater than 0.5 percent oxygen just prior to said sealing step.
During testing of existing manufacturing lines in which the amount of oxygen to which the mold sections was exposed was increased above the amounts that had previously been strictly regulated and kept below 0.5% oxygen, it was discovered that the deleterious effect on contact lens polymer properties due to exposing the contact lens mold halves to O2 did not occur, and that surprisingly the number of delamination sites that formed in the contact lens surface was significantly reduced.
Delamination sites are defects formed on the surface of the contact lens, and are surface voids where no contact lens material is present. The delamination sites or voids are typically 5-20 microns in depth and may cover large portions of the contact lens surface. When the contact lenses are magnified and imaged during the inspection process, the delamination sites look like puddles, so they may be referred to as puddles. Puddles are formed during the reaction or polymerization of the reaction mixture, e.g. polymerizable mixture, to form the contact lens polymer. It is hypothesized that a puddle is formed when the reaction mixture reacts and shrinks quickly away from the surface of a mold section. Eventhough the mold sections may be designed to accommodate some shrinkage of the reaction mixture, the reaction mixture shrinks faster than the mold sections can accomodate. Puddles are not defects formed during the demold step (when the contact lens is removed from the contact lens mold), because they are present on the contact lens after polymerization of the reaction mixture which occurs before the demold step.
After discovering that oxygen present in and on the mold sections was beneficial, testing of various manufacturing conditions was performed to quantify how much oxygen was needed to provide reduced puddles and yet avoid the deleterious effects of the oxygen on the contact lens properties.
By controlling the time and/or concentration of O2 in the gaseous mixture to which contact lens mold halves are exposed, the percent of contact lenses having puddle defects has decreased from in some cases over 40% to less than 1%. Contact lenses with puddle defects are discarded in the inspection step during their manufacture. A decrease in the discarded lenses leads to an increased yield, which has a significant impact on the manufacturing costs per contact lens. Additionally, increasing the amount of oxygen present in the enclosed volumes in the production line to the levels specified by this invention will decrease the amount of inert gas, N2 or other inert gas, which also leads to cost reductions.